One configuration of an illumination circuit of a discharge lamp (e.g., a metal halide lamp) includes a DC power supply circuit having a DC-DC converter, a DC-AC conversion circuit and a starter circuit. For example, a DC voltage output from a battery may be converted to a desired voltage in the DC power supply circuit, and is converted to an AC output in a subsequent DC-AC conversion circuit. A start-up signal (a so-called starter pulse) is superposed on the AC output, and a resultant signal is supplied to the discharge lamp (see, e.g., Japanese patent document JP-A-7-142182).
However, where a voltage is converted through two stages (i.e., a DC-DC voltage conversion and a DC-AC conversion), a circuit becomes unsuitable for miniaturization as it becomes larger in scale. Therefore, there is employed a configuration in which an output boosted through voltage conversion effected in a first stage of the DC-AC circuit is supplied to a discharge lamp (see, e.g., Japanese patent document JP-A-7-169583).
The foregoing configuration can suffer from a problem when the frequency of a starter circuit of the discharge lamp is increased.
For instance, when a secondary winding of a transformer (a so-called a starter transformer) constituting the starter circuit is connected in series to a discharge lamp, a loss in the transformer is increased when an output frequency of a DC-AC conversion circuit is increased, which, in turn, is responsible for a decrease in efficiency. An increase in an operating frequency (i.e., an operating frequency of a switching element constituting the DC-AC conversion circuit) is desirable for reducing the size of a circuit. Minimizing power loss in the starter circuit or the like also is desirable. A configuration in which the starter circuit effects boosting without use of a transformer can result in drawbacks such as a complicated circuit configuration being responsible for a cost hike or hindering miniaturization.